Moving blade for a high pressure turbine, the blade having a trailing edge of improved thermal behavior

ABSTRACT

A moving blade for a high pressure turbine of a turbomachine, the blade having at least one cooling circuit comprising at least one cavity extending radially between a tip and a base of the blade, at least one air admission opening at a radial end of the cavity(ies) to feed the cooling circuit(s) with cooing air, and a plurality of slots opening out into the cavity(ies) and opening out into the concave side of the blade between the base and the tip of the blade in a manner that is substantially perpendicular to a longitudinal axis of the blade, a connection zone being provided between a slot closest to the base of the blade and a top surface of a platform defining an inside wall for a stream of combustion gas flowing through the high pressure turbine, and the moving blade further comprising an additional cooling air exhaust slot opening out into said cavity and opening out into said connection zone in line with the trailing edge.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the general field of moving blades for a high pressure turbine in a turbomachine, and more particularly it relates to slots for exhausting cooling air from the moving blades of a high pressure turbine.

[0002] In conventional manner, a turbomachine has a combustion chamber in which air and fuel are mixed together prior to being burned therein. The gas that results from this combustion flows downstream in the combustion chamber and then feeds a high pressure turbine. The high pressure turbine has one or more rows of moving blades that are circumferentially spaced apart all around the rotor of the turbine. The moving blades of the high pressure turbine are thus subjected to the very high temperatures of the combustion gases. These temperatures reach values that are well above those that can be withstood without damage by the blades which are in contact with said gases, and as a result their lifetime is limited.

[0003] In order to solve this problem, it is known that such blades can be provided with internal cooling circuits seeking to lower their temperature. By means of such circuits, cooling air, which is generally introduced into the blade via its base, passes through the blade following a path formed by cavities that are made inside the blade, and is then ejected via slots that open out into the surface of the blade in its trailing edge, as in European application EP 0 945 594, or in its concave side, as in U.S. Pat. No. 4,601,638. More precisely, these cooling air exhaust slots are generally distributed between the base and the tip of the blade in a manner that is substantially perpendicular to the longitudinal axis of the blade.

[0004] It is also known that high pressure turbine blades fitted with cooling circuits are made by molding. The locations of the cooling circuit slots are conventionally occupied by cores disposed parallel to one another inside the mold before the metal is cast. In order to avoid weakening the core during casting, the cooling air exhaust slot closest to the base of the blade is generally made of dimensions that are greater than the dimensions of the other slots, as shown in FIG. 3 of the above-mentioned US patent.

[0005] Unfortunately, in practice, the connection zone situated between the slot and the platform supporting the blade is very poorly cooled, particularly since the air exhausted via the slot tends to be deflected towards the tip of the blade because of the large dimensions of the slot and because of the centrifugal force generated by the blade rotating. This gives rise to large temperature gradients in the trailing edge which, by conduction, give rise to cracking in the vicinity of the connection zone, which cracking is particularly harmful for the lifetime of the blade.

OBJECT AND SUMMARY OF THE INVENTION

[0006] The present invention thus seeks to mitigate that drawback by proposing a moving blade for a high pressure turbine that presents a novel shape that avoids cracking. The invention also seeks to avoid degrading the general mechanical strength of the blade, which is a part that is subjected to very high levels of mechanical stress. Finally, the invention also seeks to provide a high pressure turbine for a turbomachine, which turbine is equipped with such moving blades having improved thermal behavior.

[0007] To this end, the invention provides a moving blade for a high pressure turbine of a turbomachine, the blade having at least one cooling circuit comprising at least one cavity extending radially between a tip and a base of the blade, at least one air admission opening at a radial end of the cavity(ies) to feed the cooling circuit(s) with cooing air, and a plurality of slots opening out into the cavity(ies) and opening out into the concave side of the blade between the base and the tip of the blade in a manner that is substantially perpendicular to a longitudinal axis of the blade, a connection zone being provided between a slot closest to the base of the blade and a top surface of a platform defining an inside wall for a stream of combustion gas flowing through the high pressure turbine, the blade further comprising an additional cooling air exhaust slot opening out into said cavity and opening out into said connection zone in line with the trailing edge.

[0008] As a result, the cooling air exhausted via said additional slot is guided to the vicinity of the trailing edge over the entire surface of the connection zone so as to avoid cracks appearing therein. This particular shape for the blade base makes it possible to lower the local temperature in said zone by about 10% while conserving the aerodynamic performance obtained by having exhaust slots in the concave side. Under such circumstances, it is no longer necessary for the closest slot of the blade to present dimensions that are greater than those of the other slots since it is now the additional slot which is subjected to the pressure effects due to casting the metal. In addition, because the closest slot has been returned to normal dimensions, the problem posed by cooling air being deflected by centrifugal force is eliminated and the resulting losses in terms of flow rate are therefore reduced. Finally, the ability of the blade to withstand the various mechanical stresses to which it is subjected is not degraded by this special shape.

[0009] The additional slot is preferably of a shape taken from the following group of shapes: oblong; rectangular; triangular; and an upside-down T-shape; and it is obtained directly by casting.

[0010] In addition, this additional slot may present an outlet section that is different from the section of its feed channel.

[0011] Advantageously, a bottom longitudinal end of the additional slot is disposed immediately above said top surface of the platform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Other characteristics and advantages of the present invention appear from the following description given with reference to the accompanying drawings which show an embodiment having no limiting character. In the figures:

[0013]FIG. 1 is a perspective view of a moving blade of the invention for a high pressure turbine;

[0014]FIG. 2 is a fragmentary view on a larger scale than FIG. 1 showing a first embodiment of the additional slot for exhausting cooling air that is provided in the connection zone between the base of the blade and the platform;

[0015]FIGS. 2A and 2B are end views of the trailing edge in the vicinity of the connection zone with the platform for two variant embodiments of the additional cooling air exhaust slot;

[0016]FIG. 2C is a section view on plane AA of FIG. 2A;

[0017]FIG. 3 is a fragmentary view on a larger scale than FIG. 1 showing a second embodiment of the additional slot for exhausting cooling air; and

[0018]FIG. 4 is a view of the base of a blade and the connection zone between said base and the platform in a prior art blade.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0019]FIG. 1 is a perspective view of a moving blade 10 in accordance with the present invention, e.g. for a high pressure turbine in a turbomachine. This blade has a longitudinal axis X-X and it is fixed to a rotor disk (not shown) of the high pressure turbine by means of a root 12 that is generally in the shape of a fir-tree. It typically comprises a base 14A, a tip 14B, a concave wall 16A, a convex wall 16B, a leading edge 18, and a trailing edge 20. The root 12 joins the base 14A of the blade at a platform 22 defining an inside wall for the stream of combustion gas flowing through the high pressure turbine.

[0020] Such a blade is subjected to the very high temperatures of the combustion gases and it therefore needs to be cooled. For this purpose, and in known manner, the moving blade 10 has at least one internal cooling circuit made up, for example, of at least one cavity 24 extending radially between the base 14A and the tip 14B of the blade. This cavity is fed with cooling air via one of its radial ends by means of an air admission opening (not shown). This air admission opening is generally provided in the root 12 of the blade. A plurality of slots 26 are also provided that open out both into the cavity 24 and, in the example shown, into the concave side 16A of the blade so as to exhaust the cooling air flowing in the cavity. This disposition of the exhaust slot directly through the concave wall 16A of the blade is preferable to a disposition in the trailing edge (which requires the blade to be of greater thickness), because of the improved aerodynamic performance that it makes possible. The cooling air exhaust slots 26 are typically distributed between the base 14A and the tip 14B of the blade in a manner that is substantially perpendicular to the longitudinal axis X-X of the blade. More particularly, the slot 28 closest to the base 14A of the blade 10 is formed immediately above a connection zone 30 between the base 14A of the blade and a top surface 22A of the platform 22 beside the stream of flowing combustion gases.

[0021] The perspective view of FIG. 2 and FIGS. 2A to 2C show more clearly the shape of the trailing edge 20 of the blade in said connection zone and including the slot 28 that is closest to the base 14A of the blade 10.

[0022] In accordance with the invention, an additional slot is provided that opens out into the cavity 24 and also into said connection zone in line with the trailing edge 20. This additional slot 32 makes it possible to provide cooling by convection of this portion of the connection zone, and by conduction of the slot that is closest to the base of the blade and also of the portion of the platform that is situated as an extension of the trailing edge. Thus, by making temperature more uniform in this vicinity, it is possible to eliminate all hot spots and thus to improve the thermal behavior of the blade. The cooling air exhausted through this additional slot covers all of the surface of the connection zone in the vicinity of the trailing edge and it lowers local temperature by about 10%, doing this without modifying the aerodynamic behavior that is achieved by using exhaust slots in the concave side. Thus, any risk of cracking in the vicinity of this connection zone between the base of the blade and the platform disappears and the lifetime of the blade is lengthened. Under such circumstances, it is no longer necessary for the slot 28 closest to the base of the blade to present dimensions greater than those of the other slots 26, as illustrated by the prior art of FIG. 4, and because it can be of normal dimensions, the problem posed by cooling air being deflected in the vicinity of this slot due to centrifugal force is eliminated as are losses in terms of the resulting flow rate.

[0023] The additional slot may be of various shapes that are determined as a function of desired mechanical and thermal dimensioning criteria. Similarly, the feed channel and the outlet orifice can have sections that are different. Thus, FIG. 2 shows an additional slot presenting a feed channel and an outlet orifice that are elongate in shape, advantageously being oblong (it is also possible to envisage using a rectangular section). In FIG. 2A, the outlet section is still oblong, but the internal feed channel is substantially triangular, i.e. it flares on either side of the outlet hole so as to provide better cooling of the connection zone while also delivering a well-calibrated outlet flow rate. Depending on the thermal stresses and the mechanical forces to which the blade is subjected, this flare of varying angle may be asymmetrical, and can thus be directed towards one side only, as shown in FIG. 2B.

[0024] A variant embodiment of the additional slot in which the feed channel is of an upside-down T-shape that coincides with the shape of the outlet orifice 34 is also shown in FIG. 3.

[0025] This slot is advantageously obtained directly by casting simultaneously with the blade itself (as opposed to being machined subsequently) and it passes through the blade as far as the cavity 24 while remaining constantly immediately above the level of the top surface of the platform (i.e. its longitudinal bottom end 32A or 34A is above said top surface 22A). As a result, it is this additional slot which is subjected to the effects of the casting pressure of the metal and it is not the slot 28 closest to the base of the blade that is subjected thereto as in the prior art, so any risk of the blade being weakened because of machining is avoided. 

What is claimed is: 1/ A moving blade for a high pressure turbine of a turbomachine, the blade having at least one cooling circuit comprising at least one cavity extending radially between a tip and a base of the blade, at least one air admission opening at a radial end of the cavity(ies) to feed the cooling circuit(s) with cooing air, and a plurality of slots opening out into the cavity(ies) and opening out into the concave side of the blade between the base and the tip of the blade in a manner that is substantially perpendicular to a longitudinal axis of the blade, a connection zone being provided between a slot closest to the base of the blade and a top surface of a platform defining an inside wall for a stream of combustion gas flowing through the high pressure turbine, the blade further comprising an additional cooling air exhaust slot opening out into said cavity and opening out into said connection zone in line with the trailing edge. 2/ A blade according to claim 1, wherein said additional slot presents a shape taken from the following group of shapes: oblong; rectangular; triangular; and an upside-down T-shape. 3/ A blade according to claim 2, wherein said additional slot presents an outlet section which is different from the section of its feed channel. 4/ A blade according to claim 1, wherein said additional slot is obtained directly by casting. 5/ A blade according to claim 1, wherein a longitudinal bottom end of the additional slot is disposed immediately above said top surface of the platform. 6/ A high pressure turbine of a turbomachine, the turbine including a plurality of moving blades according to claim
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